Method for growing p-n heterojunction-based structures utilizing HVPE techniques

ABSTRACT

A method for fabricating p-type, i-type, and n-type III-V compound materials using HVPE techniques is provided. If desired, these materials can be grown directly onto the surface of a substrate without the inclusion of a low temperature buffer layer. By growing multiple layers of differing conductivity, a variety of different device structures can be fabricated including simple p-n homojunction and heterojunction structures as well as more complex structures in which the p-n junction, either homojunction or heterojunction, is interposed between a pair of wide band gap material layers. The provided method can also be used to fabricate a device in which a non-continuous quantum dot layer is grown within the p-n junction. The quantum dot layer is comprised of a plurality of quantum dot regions, each of which is typically between approximately 20 and 30 Angstroms per axis. The quantum dot layer is preferably comprised of Al x B y In z Ga 1−x−y−z N, InGaN 1−a−b P a As b , or Al x B y In z Ga 1−x−y−z N 1−a−b P a As b .

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/638,638, filed Aug. 14, 2000, which is adivisional of U.S. patent application Ser. No. 09/195,217 filed Nov. 18,1998 is now U.S. Pat. No. 6,218,269, which claims priority from U.S.patent application Ser. No. 60/066,940 filed Nov. 18, 1997, thedisclosures of which are incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor devices and,more particularly, to a method for fabricating p-n and p-i-njunction-based devices utilizing III-V compound layers as well as thestructures that result from the application of the method.

BACKGROUND OF THE INVENTION

III-V compounds such as GaN, AlN, AlGaN, and InAlGaN have uniquephysical and electronic properties that make them ideal candidates for avariety of electronic and opto-electronic devices. In particular, thesematerials exhibit a direct band gap structure, high electric fieldbreakdown, and high thermal conductivity. Additionally, materials suchas In_(x)Al_(1−x)GaN can be used to cover a wide range of band gapenergies, i.e., from 1.9 eV (where x equals 1) to 6.2 eV (where x equals0). Unfortunately, although the III-V compounds are attractive forsemiconductor applications due to their physical and electronicproperties, until recently the development of devices based on III-Vcompounds has been limited by the lack of material with suitableconductivity, specifically p-type material.

In the late 1980's, p-type GaN was grown, followed rapidly by thedevelopment of fabrication techniques for p-type AlGaN material. Thesematerials were grown using metal organic chemical vapor deposition(MOCVD) techniques and, to a lesser extent, using molecular beam epitaxy(MBE) techniques. Since the development of p-type III-V material, avariety of semiconductor devices employing both p-n and p-i-n junctionshave been demonstrated, including light emitting diodes (LEDs), laserdiodes, and photo-detectors.

In the MOCVD technique, III-V compounds are grown from the vapor phaseusing metal organic gases as sources of the Group III metals. Forexample, typically trimethylaluminum (TMA) is used as the aluminumsource and trimethylgallium (TMG) is used as the gallium source. Ammoniais usually used as the nitrogen source. In order to control theelectrical conductivity of the grown material, electrically activeimpurities are introduced into the reaction chamber during materialgrowth. Undoped III-V compounds normally exhibit n-type conductivity,the value of the n-type conductivity being controlled by theintroduction of a silicon impurity in the form of silane gas into thereaction chamber during growth. In order to obtain p-type material usingthis technique, a magnesium impurity in the form ofbiscyclopentadienymagnesium is introduced into the reactor chamberduring the growth cycle. As Mg doped material grown by MOCVD is highlyresistive, a high temperature post-growth anneal in a nitrogenatmosphere is required in order to activate the p-type conductivity.

Although the MOCVD technique has proven adequate for a variety ofcommercial devices, it has a number of limitations that constrain theusefulness of this approach. First, due to the complexity of the varioussources (e.g., trimethylaluminum, trimethylgallium, andbiscyclopentiadienylmagnesium), the process can be very expensive andone which requires relatively complex equipment. Second, the MOCVDtechnique does not provide for a growth rate of greater than a fewmicrons per hour, thus requiring long growth runs. The slow growth rateis especially problematic for device structures that require thicklayers such as high voltage rectifier diodes that often have a baseregion thickness of approximately 30 microns. Third, n-type AlGaN layersgrown by MOCVD are insulating if the concentration of AlN is high (>50mol. %). Accordingly, the concentration of AlN in the III-V compoundlayers forming the p-n junction is limited. Fourth, in order to grow ahigh-quality III-V compound material on a substrate, the MOCVD techniquetypically requires the growth of a low temperature buffer layerin-between the substrate and III-V compound layer. Fifth, generally inorder to obtain p-type III-V material using MOCVD techniques, apost-growth annealing step is required.

Hydride vapor phase epitaxy or HVPE is another technique that has beeninvestigated for use in the fabrication of III-V compound materials.This technique offers advantages in growth rate, simplicity and cost aswell as the ability to grow a III-V compound layer directly onto asubstrate without the inclusion of a low temperature buffer layer. Inthis technique III-V compounds are epitaxially grown on heatedsubstrates. The metals comprising the III-V layers are transported asgaseous metal halides to the reaction zone of the HVPE reactor.Accordingly, gallium and aluminum metals are used as source materials.Due to the high growth rates associated with this technique (i.e., up to100 microns per hour), thick III-V compound layers can be grown.

The HVPE method is convenient for mass production of semiconductordevices due to its low cost, flexibility of growth conditions, and goodreproducibility. Recently, significant progress has been achieved inHVPE growth of III-V compound semiconductor materials. AlGaN and AlNlayers have been grown as well as AlGaN/GaN heterostructures using thistechnique. The AlGaN alloys grown by HVPE have been found to beelectrically conductive up to 70 mol. % of AlN. Furthermore, since thistechnique does not require low temperature buffer layers, diodes withn-GaN/p-SiC heterojunctions have been fabricated with HVPE.

In order to fully utilize HVPE in the development and fabrication ofIII-V compound semiconductor devices, p-type layers must be produced.Conventional HVPE techniques have been unable, however, to grow suchmaterial. For example, if a magnesium acceptor impurity is added to aIII-V layer grown utilizing conventional HVPE techniques, the resultantmaterial is insulating (i.e., i-type) rather than being p-type. As aresult, the potential of the HVPE technique for fabricating p-n or p-i-njunction devices has not been realized.

Accordingly, what is needed in the art is a method for fabricatingp-type III-V compounds using the HVPE technique. The present inventionprovides such a method as well as a variety of structures realizable dueto the ability to fabricate p-type III-V compounds using HVPE.

SUMMARY OF THE INVENTION

The present invention provides a method for fabricating p-type, i-type,and n-type III-V compound materials using HVPE techniques. If desired,these materials can be grown directly onto the surface of a substratewithout the inclusion of a low temperature buffer layer. By growingmultiple layers of differing conductivity, a variety of differentdevices can be fabricated using the invention.

According to the invention, one or more Group III sources are locatedwithin one or more source zones of a furnace where they are heated to asuitable temperature, typically between 750° and 1050° C. The Group IIIsources are in the form of pure metals, e.g., gallium and aluminum, andare preferably located on or within sapphire boats. A suitable substratesuch as sapphire, silicon carbide, or gallium nitride is located withinthe growth zone of the furnace and heated to a temperature of between800° and 1200° C., and preferably between 1000° and 1100° C. In order toachieve material growth, the Group III source(s) is exposed to a halidereactive gas, thereby causing the formation of one or more halide metalcompounds (e.g., gallium chloride, aluminum trichloride, etc.). Thehalide metal compound(s) is transported to the growth zone as is areactive gas such as ammonia containing the desired Group V element. Asa result of the reaction between the reactive gas and the halide metalcompound(s), a layer of single crystal epitaxial III-V compound materialis grown. In order to achieve p-type conductivity, one or more acceptorimpurity metals are transported to the growth zone during materialgrowth, the transport rate achieving an acceptor concentration in therange of 10¹⁸ to 10²¹ atoms per cubic centimeter, and preferably withinthe range of 10¹⁹ to 10²⁰ atoms per cubic centimeter. Suitable acceptorimpurity metals include magnesium, zinc, and magnesium-zinc alloys.Preferably the acceptor impurity metals are located on or withinsapphire boats.

In at least one embodiment, the method of the invention is used tofabricate a simple p-n homojunction device. This device is comprised ofan n-type III-V compound layer, e.g., GaN or AlGaN, grown directly ontothe surface of a substrate without the inclusion of a low temperaturebuffer layer. A suitable p-type III-V compound layer is grown on then-type layer, thus achieving the p-n homojunction. Contacts aredeposited onto the structure, allowing for the conductivity of theselected substrate.

In at least one other embodiment, the method of the invention is used tofabricate a simple p-n heterojunction device, the heterojunctioncomprised of a GaN layer and an AlGaN layer. An example of a suitabledevice includes an n-type GaN layer and a p-type AlGaN layer.

In at least one other embodiment, the method of the invention is used tofabricate a double heterojunction device in which the light emittinglayer is interposed between two layers, each of which has a wider bandgap than the light emitting layer. As a result of the wide band gaplayers, the device efficiently uses injected carriers. An example of asuitable device includes an n-type GaN layer between an n-type AlGaNlayer and a p-type AlGaN layer.

In at least one other embodiment, the method of the invention is used tofabricate a device in which a p-n homojunction is interposed betweenlayers of wide band gap material. The use of a p-n homojunction allowsthe device to achieve high recombination efficiencies while thesurrounding wide band gap materials insure efficient use of injectedcarriers.

In at least one other embodiment, the method of the invention is used tofabricate a device utilizing a non-continuous quantum dot layer grownwithin the device's p-n junction. The quantum dot layer is comprised ofa plurality of quantum dot regions, each of which is typically betweenapproximately 20 and 30 Angstroms per axis. The quantum dot layer ispreferably comprised of Al_(x)B_(y)In_(z)Ga_(1−x−y−z)N,InGaN_(1−a−b)P_(a)As_(b), orAl_(x)B_(y)In_(z)Ga_(1−x−y−z)N_(1−a−b)P_(a)As_(b). The p-n junction ispreferably a homojunction consisting of layers of GaN or InGaN material.The p-n junction is preferably interposed between a pair of wide bandgap materials.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a horizontal furnace as used withthe invention;

FIG. 2 is a schematic illustration of a horizontal furnace as used togrow an n-type III-V compound layer;

FIG. 3 illustrates the method of growing an n-type III-V compound layeraccording to the invention;

FIG. 4 is a schematic illustration of a horizontal furnace as used togrow a p-type III-V compound layer;

FIG. 5 illustrates the method of growing a p-type III-V compound layeraccording to the invention;

FIG. 6 is a schematic illustration of a horizontal furnace as used togrow AlGaN layers;

FIG. 7 illustrates the method of growing AlGaN layers according to theinvention;

FIG. 8 illustrates the Mg atomic depth profile for a GaN p-n junctiongrown in accordance with the invention;

FIG. 9 illustrates C—V measurements performed on GaN p-n junctions grownin accordance with the invention;

FIG. 10 illustrates a homojunction structure comprised of an n-type GaNlayer and a p-type GaN layer;

FIG. 11 illustrates a homojunction structure comprised of an n-type GaNlayer and a p-type GaN layer utilizing a non-conductive substrate;

FIG. 12 illustrates a heterojunction structure comprised of an n-typeAlGaN layer and a p-type GaN layer;

FIG. 13 illustrates a homojunction structure comprised of an n-typeAlGaN layer and a p-type AlGaN layer;

FIG. 14 illustrates a heterojunction structure comprised of an n-typeGaN layer and a p-type AlGaN layer;

FIG. 15 illustrates a structure comprised of a narrow band gap materialinterposed between a pair of wide band gap materials;

FIG. 16 illustrates a low voltage drop device similar to the structureshown in FIG. 15;

FIG. 17 illustrates a device comprised of a p-n homojunction interposedbetween a pair of wide band gap material layers;

FIG. 18 illustrates an alternate device comprised of a p-n homojunctioninterposed between a pair of wide band gap material layers;

FIG. 19 illustrates yet another alternate device comprised of a p-nhomojunction interposed between a pair of wide band gap material layers;

FIG. 20 illustrates yet another alternate device comprised of a p-nhomojunction interposed between a pair of wide band gap material layers;

FIG. 21 illustrates a device comprised of a non-continuous quantum dotlayer within a device's p-n junction;

FIG. 22 illustrates an alternate device comprised of a non-continuousquantum dot layer within a device's p-n junction;

FIG. 23 illustrates yet another alternate device comprised of anon-continuous quantum dot layer within a device's p-n junction; and

FIG. 24 illustrates yet another alternate device comprised of anon-continuous quantum dot layer within a device's p-n junction.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention provides a method and apparatus for producingp-type III-V nitride compound materials utilizing HVPE techniques. As aresult of the ability to fabricate such p-type material, the presentinvention allows a variety of device structures to be realized as well.

Processes

FIG. 1 is a schematic illustration of a horizontal furnace as used withthe invention. It should be understood that the invention is not limitedto this particular furnace configuration as other furnace configurations(e.g., vertical furnaces) that offer the required control over thetemperature, temperature zones, gas flow, source location, substratelocation, etc., can also be used. Furnace 100 is comprised of multipletemperature zones, preferably obtained by using multiple heaters 101,each of which at least partially surrounds furnace tube 103. Withinfurnace tube 103 are multiple source tubes 105. Although source tubesare used in the preferred embodiment of the invention, other means ofseparating the sources can be used, such as furnace partitions.

Within each source tube 105 is a source boat 107. As used herein, theterm “boat” simply refers to a means of holding the source material.Therefore boat 107 may simply be a portion of a tube with an outerdiameter that is slightly smaller than the inner diameter of thecorresponding source tube 105. Alternately, boat 107 may be comprised ofa plate of suitable material that fits within tube 105. Alternate boatconfigurations are known by those of skill in the art.

A control rod 109 determines the position of each boat 107 withinfurnace 103. Control rods 109 may be manually manipulated, as providedfor in the illustrated configuration, or coupled to a roboticpositioning system. Coupled to each source tube 105 is a gas source 111.The rate of gas flow through tubes 105 is controlled by valves 113 thatmay be manually controlled or controlled via an automated system (notshown).

A substrate 115 is located within the growth zone of furnace 103.Depending upon the desired throughput of furnace 103, substrate 115 maybe either manually or automatically positioned within furnace 103.Additionally, substrates can be processed singularly or in batches. Inaccordance with the invention, substrate 115 may be comprised ofsapphire (Al₂O₃), silicon carbide (SiC), gallium nitride (GaN), or othersuitable single crystal material. Substrate 115 can be produced by anyconventional technique.

The individual sources 117 held within boats 107 are determined by thedesired composition of the layers to be grown on substrate 115. Forexample, to grow a III-V compound layer comprised of gallium (Ga) andnitrogen (N), Ga metal is placed within one of the source boats 107. Togrow a III-V compound layer comprised of Ga, N, and aluminum (Al), Almetal is placed within a boat 107 different from that used to hold theGa metal. To form a p-type layer of GaN, a suitable dopant such asmagnesium (Mg), zinc (Zn), or a MgZn alloy, is placed within anotherboat 107 (i.e., different from the boats used to hold the other sourcematerials). If multiple dopants are used, for example both Mg and Zn,the dopants may be in the form of an alloy, and thus be located within asingle boat, or be in the form of individual materials, and thereforepreferably located within separate boats.

FIGS. 2 and 3 illustrate a specific reactor 201 and the steps used togrow an n-type III-V compound layer of GaN, respectively. Althoughreactor 201 is a hot-wall, horizontal reactor and the process is carriedout in an inert gas flow at atmospheric pressure, as previously notedother reactor configurations can be used to perform the HVPE growthtechnique of the present invention. Within reactor 201 is a source tube203, preferably comprised of quartz. A boat 205, comprised of a suitablematerial, contains a Ga metal source 207. Source tube 203 is coupled toa supply 209 of a halide reactive gas, preferably HCl. A source ofammonia gas 211 and a source of an inert gas such as argon (Ar) 213, arealso coupled to reactor 201. Substrate 115 is preferably located on apedestal 215, comprised of a suitable material such as quartz.Preferably the substrates are cleaned using an etching process prior toinitiating any III-V compound layer growth.

Initially reactor 201 is filled with Ar gas (301). The flow of Ar gas,controlled by a metering valve 217, is in the range of 1 to 25 litersper minute. Substrate (or substrates) 115 is then heated to the desiredgrowth temperature (step 303), preferably in the range of 800° to 1200°C., and more preferably to a temperature of between 1000° and 1100° C.As noted above, in preparation for growing III-V compound layers,typically substrate(s) 115 is etched to remove residual surfacecontamination, for example using gaseous HCl from supply 209. Ga sourcematerial 207 is heated to a temperature of 750° to 1050° C. (step 305).Gaseous HCl 209 is then introduced into source tube 203 (step 307),resulting in the HCl reacting with the Ga to form gallium chloride. Thegallium chloride is delivered to the growth zone by the flow of Ar gas(step 309). Simultaneously, ammonia gas 211 is delivered to the growthzone (step 311). As a result of the reaction between the galliumchloride and the ammonia (step 313), a layer of single crystal epitaxialn-type GaN is grown on the surface of substrate 115 (step 315). Thegrowth rate of the GaN ranges from 0.05 to 1 micron per minute, the rateprimarily being dependent upon the flow rate of HCl. After completion ofthe desired layer, and assuming no additional layers are required, theflow of HCl and ammonia gas is stopped and substrate 115 is cooled inthe flowing Ar gas (step 317).

Although not shown in FIGS. 2 and 3, the ratio of donors to acceptorscan be further controlled by adding donor impurities to the material asthe n-type layer is being grown. Suitable donor materials include, butare not limited to, oxygen (O), silicon (Si), germanium (Ge), and tin(Sn).

As illustrated in FIGS. 4 and 5, by slightly varying the apparatus shownin FIG. 2 and the process of FIG. 3, p-type GaN can be grown on thesurface of substrate 115. It is understood, and is illustrated furtherbelow, that one or more layers of different composition can beinterposed between substrate 115 and the p-type GaN layer.

As shown in FIG. 4, a second source tube 401 is included. A boat 403containing an appropriate acceptor impurity metal 405 is containedwithin tube 401, the location of which is controlled by rod 109 aspreviously described. Although acceptor impurity metal can either be apure metal (e.g., Mg or Zn) or an alloy (e.g., MgZn), it is understoodthat the grown III-V layer can be doped with multiple acceptor impuritymetals, each of which is contained within a separate source tube.

The process used to grow the p-type GaN layer is similar to thatpreviously described except that acceptor impurity metal 405 isintroduced into the growth zone during growth of the GaN layer. Asshown, source tube 401 is coupled to the inert gas supply, in thisembodiment Ar gas 213. Acceptor impurity metal 405 is simultaneouslyheated with Ga source 207 (step 501) to a temperature in the range of250° to 1050° C. If acceptor impurity metal 405 is comprised of Mg,preferably the temperature of the source is within the range of 450° to700° C., more preferably within the range of 550° to 650° C., and stillmore preferably to a temperature of approximately 615° C. Prior toinitiating growth, preferably acceptor impurity metal 405 is etched, forexample using HCl gas 209, thereby insuring minimal sourcecontamination. During growth, Ar gas is passed through source tube 401at a relatively high flow rate, preferably between 1000 and 4000standard cubic centimeters per minute, and more preferably between 2000and 3500 standard cubic centimeters per minute. Due to the flow of Argas, atoms of the acceptor impurity metal are delivered to the growthzone (step 503) and incorporated into the GaN material growing either onsubstrate 115 or on a previously grown layer within the growth zone(step 505).

Although the above described process yields p-type GaN material, theinventors have found that an annealing step (step 507) can be used tofurther improve the properties of this layer, specifically lowering theresistivity of the p-type layer. Preferably the annealing step isperformed immediately after the growth of the p-type layer is completed.In the preferred embodiment, the material is annealed for approximately10 minutes in nitrogen at a temperature within the range of 700° to 800°C. The annealing step helps to drive the hydrogen out of the layer. Itis understood that other annealing temperatures and times can used, forexample, annealing at a lower temperature for an extended period oftime. It is also understood, as previously described, that annealingstep 507 is not required to achieve p-type III-V material according tothe invention.

In addition to n-type and p-type III-V compound layers, insulating(i-type) III-V layers can be grown using the present invention. Theprocess is similar to that described above, except that during growth ofthe III-V material, fewer atoms of the acceptor impurity metal aredelivered to the growth zone, thereby leading to a lower doping level.If required, donor impurities can be delivered to the growth zone aswell.

It is understood that although only III-V layers of GaN are describedabove, other III-V compound layers can be grown using the presentinvention. For example, an apparatus and process for growing AlGaNlayers is illustrated in FIGS. 6 and 7. The illustrated reactorconfiguration is similar to that shown in FIG. 4 with the addition ofsource tube 601. Within source tube 601 is an Al source 603, containedwithin a boat 605. An acceptor impurity source 405 is also provided,thus allowing reactor 600 to grow AlGaN layers of n-type, i-type, orp-type simply by controlling the level of acceptor doping. It should beunderstood that by controlling Al source 603, reactor 600 can be used togrow GaN layers as well as AlGaN. As previously noted, controlling thelevel of acceptor doping allows different conductivities to be obtainedfor the GaN (or AlGaN) layers.

As illustrated in FIG. 7, the process used to grow AlGaN layers is quitesimilar to the GaN process previously described. In particular, afterthe reactor has been flushed and filled with inert gas (step 301), thesubstrate is heated (step 303) as well as all of the sources (step 701).Aluminum source 603 is heated to a temperature within the range of 700°and 850° C. Altering the position of a given source within the furnaceusing control rods 109 provides temperature control for that specificsource. To grow an AlGaN layer, HCl gas 209 is introduced into Ga sourcetube 203 and Al source tube 601 (step 703), resulting in the formationof gallium chloride and aluminum trichloride which is delivered to thegrowth zone by the flow of Ar gas 213 (step 705). If p-type material isdesired, Ar gas also flows through tube 401 containing acceptor impuritymetal 405 (e.g., Mg, Zn, MgZn, etc.) (step 503, shown in phantom).Ammonia gas 211 is introduced into the growth zone (step 311)simultaneously with delivery of the source materials. As a result of thereaction between gallium chloride, aluminum trichloride, and the ammoniagas (step 707), AlGaN is grown. Depending upon the concentration, ifany, of acceptor impurities, the AlGaN layer may be n-type, i-type, orp-type. If p-type material is grown, and as previously noted, anannealing step (step 507, shown in phantom) can be added to furtherimprove the characteristics of the p-type material. In the preferredembodiment of the invention, preferably the p-type AlGaN is of the formAl_(x)Ga_(1−x)N where x is equal to 0.1. Other aluminum concentrationscan be used, however, although in general the relative concentration ofaluminum cannot exceed 0.2 (i.e., Al_(0.2)Ga_(0.8)N). As in the previousexamples, the growth rate is between 0.05 and 1 micron per minute, andmore typically between 0.1 and 0.5 microns per minute, the rateprimarily being dependent upon HCl flow rates.

It will be understood that the descriptions provided above with respectto the growth of specific composition layers is meant to beillustrative, and not limited, of the invention. Additionally it will beunderstood that multiple layers of differing composition can be grownusing the present invention. For example, a layer of n-type GaN can begrown followed by a layer of p-type GaN, the second layer simplyrequiring sufficient doping with an acceptor impurity metal duringgrowth. Accordingly, by controlling the delivery of multiple sources tothe growth zone, a multi-layer device can be grown.

FIG. 8 shows the Mg atomic depth profile measured by secondary ion massspectroscopy (SIMS) for a GaN p-n junction grown in accordance with theinvention. The base layer of this junction was n-type GaN on top ofwhich was grown a Mg doped p-type GaN layer. Both layers were grown inthe same epitaxial run. Background electron carrier concentration in then-type GaN layers was less than 10¹⁷ atoms per cubic centimeter.

As previously described, the level of doping controls the conductivityof the III-V compound layer. In order to achieve p-type material, it isnecessary for the acceptor concentration (N_(a)) to be greater than thedonor concentration (N_(d)). The inventors have found that in order toachieve the desired N_(a)/N_(d) ratio and grow p-type III-V material(e.g., GaN or AlGaN), the concentration of the acceptor impurity metal(e.g., Mg) must be in the range of 10¹⁸ to 10²¹ atoms per cubiccentimeter, and more preferably in the range of 10¹⁹ to 10²⁰ atoms percubic centimeter. For an i-type layer, the doping level must bedecreased, typically such that the dopant concentration does not exceed10¹⁹ atoms per cubic centimeter.

GaN layers of both n-type and p-type grown on SiC substrates using theprocess of the invention were characterized by x-ray diffraction. Asmeasured, the layers exhibited narrow x-ray rocking ω-scan curves (e.g.,<150 arc sec). Additionally, the electrical characteristics of GaN p-ndiodes were measured. The current densities were less than 5·10⁻² ampsper square centimeter with an applied reverse voltage of up to 10 volts.The forward I-V characteristics of the GaN p-n junctions at both highand low current density were measured for various ambient temperatures.The mesa diameter was approximately 1.3·10⁴ square centimeters. The Mgconcentration of the p-type GaN, measured by SIMS, was 2.5·10²⁰ atomsper cubic centimeter. Background donor concentration in the undoped GaNlayer was approximately 1·10¹⁷ atoms per cubic centimeter. The measurednonideality factor was 3.2, 2.8, 2.3, and 2.0 for 20° C., 70° C., 140°C., and 215° C., respectively. Accordingly, the p-n junctions operatednot only at room temperature, but also at higher temperatures. The C—Vmeasurements, illustrated in FIG. 9, were performed at a frequency of 1MHz on mesa structures with an area of approximately 3.6·10⁻⁵ squarecentimeters. As shown, the GaN p-n junctions demonstrated a voltagecut-off of about 2.5 volts.

In order to grow p-type III-V material, the inventors have found that inaddition to using extremely pure source material (e.g., 99.999 to99.9999 purity Mg) and pre-etching the material to insure minimalsurface contamination, preferably the source material is containedwithin a sapphire boat. It is believed that the sapphire boat aids inthe suppression of donors, for example arising from oxygen, within theas-grown layer. The suppression of donors may be due to the eliminationof boat/source reactions resulting from the use of a sapphire boat. Theinventors have also found that in order to grow p-type AlGaN material,the aluminum source material is preferably contained within a siliconcarbide boat. Additionally, in the preferred embodiment of theinvention, sapphire boats are used with all of the other sources,thereby insuring minimal donor contamination.

In addition to requiring the use of non-reactive boats, e.g., sapphireboats in the preferred embodiment, and specific acceptor impurity sourcetemperatures, e.g., preferably within the range of 450 to 700° C. forMg, and more preferably within the range of 550 to 650° C. for Mg,reactor pre-conditioning has been found to be beneficial, and in someinstances required, in order to obtain p-type III-V compound growth.Specifically, the inventors have found that using a conventionalhorizontal reactor under typical conditions, the reactor and sourcetubes must be pre-conditioned by saturating them with a sufficientquantity of acceptor impurity materials prior to growing p-type III-Vcompound materials. The pre-conditioning process may be accomplished byperforming multiple conventional acceptor impurity doping runs until thereactor tube and the source tubes are sufficiently saturated.Alternately, one or two high saturation doping runs may be performed inwhich a large quantity of the acceptor impurity is forced to flowthrough the source tube and the reactor.

In addition to finding that reactor pre-conditioning is necessary undercertain circumstances, the inventors have also found that if the reactorand the source tubes become too saturated with the acceptor impurity, itmay no longer be possible to grow p-type III-V compound materials. Whensuch over-saturation occurs, the level of acceptor impurity within thesystem must be lowered, for example by executing multiple materialgrowth runs in the absence of acceptor impurities within the sourcetubes. Alternately, the reactor may be dismantled, cleaned, reassembled,and again pre-conditioned.

One of the advantages of the present invention is that a devicestructure, e.g., a p-n junction heterostructure, can be fabricatedwithout requiring growth of low temperature buffer layers. In aconventional device comprised of III-V compound layers grown using MOCVDor a similar technique, a buffer layer must be grown on the surface ofthe substrate prior to growing epitaxial layers of the III-V nitridebased compound. The buffer layer, typically consisting of AlN, GaAlN, orGaN, is grown at a temperature less than that required for the epitaxiallayer to be formed. By growing the buffer layer at a low temperature,typically between 200° and 900° C., and more typically between 400° and800° C., a polycrystalline layer is formed. When the substratetemperature is increased to approximately 1000° C. in order to grow theepitaxial layers, the buffer layer becomes partially monocrystalline,serving as a seed crystal for the epitaxial layers.

Accordingly, as a consequence of this additional layer, conventionalIII-V device fabrication techniques require additional processing steps.

The present invention does not require the additional processing stepsdescribed above since a low temperature buffer layer is not requiredbetween the substrate and the epitaxial layers. As previously noted,according to the invention the first semiconductor layer of the desireddevice, for example an n-type GaN layer, is grown directly on thesubstrate without requiring that the substrate initially be maintainedat a lower temperature to allow buffer layer growth. For example,according to the invention the first semiconductor layer is grown at atemperature preferably greater than 900° C., more preferably greaterthan 950° C., and still more preferably greater than 1000° C.Consequently, the present invention allows both device structures andfabrication processes to be simplified due to the elimination of the lowtemperature buffer layer.

Light Emitting Structures

The following structures are meant to be illustrative, and not limiting,as to the invention. These structures are fabricated using thepreviously described HVPE processes, thus allowing III-V compound layersof the desired conductivity to be grown.

In order to fabricate light emitting structures, several related factorsare considered. First, the light emitting layer is preferably doped witha suitable impurity (e.g., O, Si, Ge, or Sn) in order to improve thelayer's light emission characteristics. Second, the design must insurethat sufficient carriers are injected into the light emitting layer ofthe device to result in light emission. As a higher concentration ofinjected carriers will typically result in a device capable of operatingat higher brightness levels, preferably the layer or layers injectingcarriers into the light emitting layer are doped with suitable dopantssuch as Mg or Zn. Additionally, the inventors have found that co-doping,for example with Mg and O or Zn and O, can be used to achieve very highcarrier concentrations.

In a homojunction device, carriers travel from the layer of highconcentration to the layer of low concentration. As previously noted,increasing the ratio of carriers in the injecting layer(s) to that ofthe light emitting layer increases the achievable brightness level of adevice. Accordingly, doping the light emitting layer to improve itslight emission characteristics can adversely affect this ratio, therebylowering the achievable brightness level. This problem can be furtherexacerbated in devices in which the intended light emitting layer is thep-type layer as this layer is already doped with various impurities(e.g., Mg or Zn) that result in an increased carrier concentration.

Although the present invention can be used to grow homojunction devices,in order to avoid the design limitations described above, preferably theinvention is used to grow heterojunction devices utilizing wide band gapmaterials. By growing a wide band gap layer such as AlGaN immediatelyadjacent to a narrow band gap layer such as GaN, carriers are forced totravel from the wide band gap material to the narrow band gap materialregardless of the carrier concentration ratio. Accordingly, greaterdesign latitude is provided as the light emitting layer can be optimallydoped without adversely affecting the device's output.

Structures 1 and 2

Homojunction structure 1000 shown in FIG. 10 is comprised of an n-typeGaN layer 1001 and a p-type GaN layer 1003. As shown, there is nointervening low temperature buffer layer between n-type GaN layer 1001and substrate 1005. As previously described, preferably substrate 1005is fabricated from Si or SiC. Contacts 1007 and 1008 are applied to theupper surface of p-type GaN layer 1003 and the lower surface ofsubstrate 1005, respectively. Contacts 1007 and 1008 are comprised of asuitable material such as nickel (Ni), palladium (Pd), gold (Au),platinum (Pt), AuNi, or PdPt.

As previously described, the p-type layer is grown by doping the layerduring growth with a suitable acceptor impurity. Preferably the acceptorimpurity is comprised of Mg, Zn, or Mg+Zn although other acceptorimpurities can be used. The n-type layer may contain small amounts ofone or more donor impurities (e.g., oxygen (O), silicon (Si), germanium(Ge), and/or tin (Sn)). As described above, the carrier concentrationratio between layers 1001 and 1003 determines which of these layersserves as the light emitting layer.

It will be understood that the above configuration assumes a conductivesubstrate, thereby allowing contact 1008 to be applied to the lowersurface of the substrate. If an insulating substrate is used, forexample one fabricated of Al₂O₃, a different contact arrangement must beused. For example, the structure shown in FIG. 11 is similar to thatshown in FIG. 10 except for the location of one of the contacts.Specifically, a contact 1101 is deposited on a portion of n-type GaNlayer 1001. Preferably etching is used to create the mesa onto whichcontact 1101 is deposited.

It should be understood that the following structures assume aconductive substrate. The approach illustrated in FIG. 11 can be used,however, in these same structures when it is desirable to use anon-conductive substrate.

Structures 3-5

FIG. 12 is an illustration of a heterojunction device 1200 utilizing ap-type GaN narrow band gap material 1201 and an n-type AlGaN wide bandgap material 1203. The structure is grown directly onto substrate 1005without inclusion of a low temperature buffer layer. Due to the use of awide band gap material, carriers are injected by n-type AlGaN layer 1203into light emitting layer 1201. In addition to doping layer 1201 with asuitable acceptor impurity as previously described, preferably layer1201 is also doped with one or more dopants (e.g., Si) that improve theemission characteristics of the device. As previously described, thep-type layer is grown by doping the layer during growth with a suitableacceptor impurity. Preferably the acceptor impurity is comprised of Mg,Zn, or Mg+Zn although other acceptor impurities can be used. N-typelayer 1203 is preferably doped with one or more donor impurities toimprove the carrier concentration of the layer. It is understood thatp-type GaN layer 1201 can be replaced with a p-type AlGaN layer 1301 asshown in FIG. 13. Device 1300, however, suffers from the same designconstraints previously described with reference to GaN homojunctiondevice 1000.

Device 1400, illustrated in FIG. 14, is similar to device 1200 exceptthat the p-type layer is comprised of the wide band gap material (e.g.,p-type AlGaN layer 1401) and the n-type layer is comprised of the narrowband gap material (e.g., n-type GaN layer 1403).

Structures 6 and 7

A problem inherent in structures 3-5 is the inefficient use of thecarriers injected into the light emitting layer. Specifically, a portionof the injected carriers may pass through the light emitting layer intothe adjoining layer or layers without causing light emission.

In order to overcome the above-identified problem, preferably the lightemitting layer is sandwiched between two layers, both of which have awider band gap than the light emitting layer. As a result of thisconfiguration, the wide band gap material on either side of the lightemitting layer prevent injected carriers from simply passing through thelayer without being effectively used by the device. Accordingly, thisdevice is more efficient than structures 3-5.

FIG. 15 illustrates one embodiment of a device in which a narrow bandgap material is interposed between a pair of wide band gap materials. Itwill be understood that this device is only meant to be illustrative ofthe described approach as other material combinations can be usedwithout departing from the approach. As shown, device 1500 utilizes alight emitting layer 1501 comprised of n-type GaN interposed between ap-type AlGaN layer 1503 and an n-type AlGaN layer 1505. As previouslynoted, AlGaN has a wider band gap than GaN.

FIG. 16 illustrates a variation of device 1500. Device 1600 includes anadditional layer 1601 interposed between p-type AlGaN layer 1503 andcontact 1007. Layer 1601, included in order to achieve a low voltagedrop device, is comprised of a narrow band gap material such as p-typeGaN.

Structures 8-11

FIGS. 17-20 illustrate a various configurations of a device, all ofwhich include a p-n homojunction interposed between layers of wide bandgap material. As a p-n homojunction has inherently less crystal defectsthan a p-n heterojunction, this device is capable of achieving higherrecombination efficiencies than a device based on a heterojunction.

FIGS. 17 and 18 illustrate two configurations for this device, bothutilizing a GaN p-n homojunction sandwiched between layers of AlGaN. Thep-n homojunction is comprised of a layer 1701 of n-type GaN and a layer1703 of p-type GaN. Adjacent to n-type GaN layer 1701 is a layer 1705 ofn-type AlGaN. Adjacent to p-type GaN layer 1703 is a layer 1707 ofp-type AlGaN. Additionally, device 1800 utilizes a p-type substrate 1709(e.g., p-type SiC).

The device configurations illustrated in FIGS. 19 and 20 are the same asthose shown in FIGS. 17 and 18 with the addition of a narrow band gapmaterial layer (i.e., layers 1901 and 2001, respectively) interposedbetween the outermost wide band gap layer and contact 1007. Aspreviously noted, the inclusion of such a layer is advantageous due tothe low voltage drop incurred by the device.

It will be understood that the light emitting layer(s) of devices1700-2000 can be comprised of layer 1701, layer 1703, or both layers1701 and 1703, depending upon the dopants used with each layer.

Structures 12-15

FIGS. 21-24 illustrate various configurations of a device, all of whichinclude a non-continuous quantum dot ‘layer’ 2101 within the device'sp-n junction. As noted, layer 2101 is non-continuous, i.e., it iscomprised of a plurality of ‘islands’, each of which is approximatelythe same size in all three dimensions. Accordingly, if layer 2101 is 20Angstroms thick, typically each quantum dot region will be 20 Angstromsby 20 Angstroms by 20 Angstroms. In general, the majority of quantum dotregions are less than 30 Angstroms in each dimension. Preferably themajority of quantum dot regions are approximately 20 Angstroms by 20Angstroms by 20 Angstroms.

Quantum dot layer 2101 is preferably comprised ofAl_(x)B_(y)InGa_(1−x−y−z)N. Preferably 0.01≦x+y≦0.2. More preferably0.01≦x+y≦0.03 and x+y+z≦0.2. Even more preferably 0.01≦x+y≦0.03 andx+y+z≦0.15.

In an alternate embodiment, quantum dot layer 2101 is comprised ofInGaN_(1−a−b)P_(a)As_(b). Preferably 0.01≦a+b≦0.2. More preferably0.01≦a+b≦0.03.

In an alternate embodiment, quantum dot layer 2101 is comprised ofAl_(x)B_(y)In_(z)Ga_(1−x−y−z)N_(1−a−b)P_(a)As_(b). Preferably0.01≦x+y≦0.2 and 0.01≦a+b≦0.2. More preferably 0.01≦x+y≦0.03, x+y+z≦0.2,and 0.01≦a+b≦0.03. Even more preferably 0.01≦x+y≦0.03, x+y+z≦0.15, and0.01≦a+b≦0.03.

Preferably the process used to grow quantum dot layer 2101 is similar tothe previously described processes except for the introduction of theadditional elements during the layer's growth. If indium (In) is added,it is simultaneously heated with the Ga source, the In being heated to atemperature within the range of 750 and 850° C. After heating, NH₃ isintroduced into the In source tube. The product of the reaction isdelivered to the growth zone by the flow of Ar gas. If boron (B),phosphorous (P), or arsenic (As) are added to the quantum dot layer,they are introduced using gaseous sources.

As shown in FIGS. 21-24, quantum dot layer 2101 is sandwiched between alayer 2103 of n-type GaN and a layer 2105 of p-type GaN. Alternately,layer 2103 can be comprised of n-type InGaN and layer 2105 can becomprised of p-type InGaN. Adjacent to n-type GaN layer 2103 (or n-typeInGaN) is a layer 2107 of n-type AlGaN. Adjacent to p-type GaN layer2105 (or p-type InGaN) is a layer 2109 of p-type AlGaN. Additionally,devices 2200 and 2400 utilize p-type substrate 1709 (e.g., p-type SiC).

The device configurations illustrated in FIGS. 23 and 24 are the same asthose shown in FIGS. 21 and 22 with the addition of a narrow band gapmaterial layer (i.e., layers 2301 and 2401, respectively) interposedbetween the outermost wide band gap layer and contact 1007. Aspreviously noted, the inclusion of such a layer is advantageous due tothe low voltage drop incurred by the device.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

What is claimed is:
 1. A method of fabricating a p-n heterojunctiondevice without the inclusion of a low temperature buffer layer, themethod utilizing HVPE techniques and comprising the steps of: locating afirst Group III metal in a first source zone of a reaction chamber;locating a second Group III metal in a second source zone of saidreaction chamber; locating at least one acceptor impurity metal in athird source zone of said reaction chamber; locating a substrate withina growth zone of said reaction chamber; heating said substrate to afirst temperature, wherein said first temperature is greater than 900°C.; heating said first Group III metal to a second temperature; heatingsaid second Group III metal to a third temperature; heating said atleast one acceptor impurity metal to a fourth temperature; introducing ahalide reaction gas into said first source zone to form a first halidemetal compound; introducing said halide reaction gas into said secondsource zone to form a second halide metal compound; transporting saidfirst and second halide metal compounds to said growth zone; introducinga reaction gas into said growth zone, said reaction gas containing atleast one Group V element; growing a first III-V layer on saidsubstrate, said first III-V layer formed by said reaction gas reactingwith said first and second halide metal compounds, wherein said firstIII-V layer is an n-type III-V layer; discontinuing said step oftransporting said second halide metal compound to said growth zone;growing a second III-V layer on said first III-V layer, said secondIII-V layer formed by said reaction gas reacting with said first halidemetal compound, wherein said second III-V layer is an n-type III-Vlayer; resuming said step of transporting said second halide metalcompound to said growth zone; transporting said at least one acceptorimpurity metal to said growth zone; and growing a third III-V layer onsaid second III-V layer, said third III-V layer formed by said reactiongas reacting with said first and second halide metal compounds, whereinsaid third III-V layer contains said at least one acceptor impuritymetal, and wherein said third III-V layer is a p-type III-V layer. 2.The method of claim 1, further comprising the steps of: depositing afirst contact on said third III-V layer; and depositing a second contacton said substrate.
 3. The method of claim 1, further comprising thesteps of: discontinuing said step of transporting said second halidemetal compound to said growth zone; and growing a fourth III-V layer onsaid third III-V layer, said fourth III-V layer formed by said reactiongas reacting with said first halide metal compound, wherein said fourthIII-V layer contains said at least one acceptor impurity metal, andwherein said fourth III-V layer is a p-type III-V layer.
 4. The methodof claim 3, further comprising the steps of: depositing a first contacton said fourth III-V layer; and depositing a second contact on saidsubstrate.
 5. The method of claim 1, further comprising the steps of:selecting Ga as said first Group III metal; selecting Al as said secondGroup III metal; and selecting Mg as said at least one acceptor impuritymetal.
 6. The method of claim 1, further comprising the step ofpositioning said at least one acceptor impurity metal on a firstsapphire boat within said third source zone.
 7. The method of claim 6,further comprising the steps of: positioning said first Group III metalon a second sapphire boat within said first source zone; and positioningsaid second Group III metal on a third sapphire boat within said secondsource zone.
 8. The method of claim 6, further comprising the steps of:positioning said first Group III metal on a second sapphire boat withinsaid first source zone; and positioning said second Group III metal on asilicon carbide boat within said second source zone.
 9. The method ofclaim 1, further comprising the steps of: locating a second acceptorimpurity metal in a fourth source zone of said reaction chamber; heatingsaid second acceptor impurity metal to a fifth temperature; andtransporting said second acceptor impurity metal to said growth zonesimultaneously with said at least one acceptor impurity metal.
 10. Themethod of claim 9, positioning said second acceptor impurity metal on asapphire boat within said fourth source zone.
 11. The method of claim 9,selecting Zn as said second acceptor impurity metal.
 12. The method ofclaim 1, further comprising the step of pre-filling said reactionchamber with a flowing inert gas.
 13. The method of claim 1, whereinsaid first temperature is within the temperature range of 1000° C. to1100° C.
 14. The method of claim 1, wherein said second and thirdtemperatures are within the temperature range of 750° C. to 1050° C. 15.The method of claim 1, wherein said fourth temperature is within thetemperature range of 450° C. to 700° C.
 16. The method of claim 1,wherein said fourth temperature is within the temperature range of 550°C. to 650° C.
 17. The method of claim 1, wherein said fourth temperatureis approximately 615° C.
 18. The method of claim 1, further comprisingthe step of annealing said third III-V layer.
 19. The method of claim18, said annealing step further comprised of the step of heating saidthird III-V layer to a temperature within the range of 700° C. to 800°C., said heating step performed within a nitrogen atmosphere.
 20. Themethod of claim 19, wherein said annealing step is performed forapproximately 10 minutes.
 21. The method of claim 1, further comprisingthe step of selecting a transport rate associated with said step oftransporting said at least one acceptor impurity metal to said growthzone, wherein said selected transport rate achieves a concentration ofsaid at least one acceptor impurity metal within said third III-V layerof between 10¹⁸ to 10²¹ atoms cm⁻¹.
 22. The method of claim 1, furthercomprising the step of selecting a transport rate associated with saidstep of transporting said at least one acceptor impurity metal to saidgrowth zone, wherein said selected transport rate achieves aconcentration of said at least one acceptor impurity metal within saidthird III-V layer of between 10¹⁹ to 10²⁰ atoms cm⁻¹.
 23. The method ofclaim 3, further comprising the step of selecting a transport rateassociated with said step of transporting said at least one acceptorimpurity metal to said growth zone, wherein said selected transport rateachieves a concentration of said at least one acceptor impurity metalwithin said fourth III-V layer of between 10¹⁸ to 10²¹ atoms cm⁻³. 24.The method of claim 3, further comprising the step of selecting atransport rate associated with said step of transporting said at leastone acceptor impurity metal to said growth zone, wherein said selectedtransport rate achieves a concentration of said at least one acceptorimpurity metal within said fourth III-V layer of between 10¹⁹ to 10²⁰atoms cm⁻³.
 25. The method of claim 1, further comprising the step ofpre-conditioning said reaction chamber.
 26. The method of claim 25,wherein said pre-conditioning step is further comprised of saturatingsaid growth zone and said first, second, and third source zones withsaid at least one acceptor impurity metal.
 27. The method of claim 1,further comprising the step of co-doping said third III-V layer with O.28. The method of claim 3, further comprising the step of co-doping saidfourth III-V layer with O.
 29. The method of claim 1, further comprisingthe step of doping said second III-V layer with at least one donorimpurity selected from the group of materials consisting of O, Si, Ge,and Sn.
 30. The method of claim 1, further comprising the step ofetching said substrate, said first Group III metal, said second GroupIII metal, and said at least one acceptor impurity metal to removesurface contamination, said etching step performed prior said firstgrowing step.
 31. The method of claim 30, wherein said etching step isperformed prior to said first transporting step.
 32. The method of claim30, wherein said etching step is performed prior to said first heatingstep.
 33. A method of fabricating a p-n heterojunction device withoutthe inclusion of a low temperature buffer layer, the method utilizingHVPE techniques and comprising the steps of: locating a first Group IIImetal in a first source zone of a reaction chamber; locating a secondGroup III metal in a second source zone of said reaction chamber;locating at least one acceptor impurity metal in a third source zone ofsaid reaction chamber; locating a p-type substrate within a growth zoneof said reaction chamber; heating said p-type substrate to a firsttemperature, wherein said first temperature is greater than 900° C.;heating said first Group III metal to a second temperature; heating saidsecond Group III metal to a third temperature; heating said at least oneacceptor impurity metal to a fourth temperature; introducing a halidereaction gas into said first source zone to form a first halide metalcompound; introducing said halide reaction gas into said second sourcezone to form a second halide metal compound; transporting said first andsecond halide metal compounds to said growth zone; transporting said atleast one acceptor impurity metal to said growth zone; introducing areaction gas into said growth zone, said reaction gas containing atleast one Group V element; growing a first III-V layer on saidsubstrate, said first III-V layer formed by said reaction gas reactingwith said first and second halide metal compounds, wherein said firstIII-V layer contains said at least one acceptor impurity metal, andwherein said first III-V layer is a p-type III-V layer; discontinuingsaid step of transporting said second halide metal compound to saidgrowth zone; growing a second III-V layer on said first III-V layer,said second III-V layer formed by said reaction gas reacting with saidfirst halide metal compound, wherein said second III-V layer containssaid at least one acceptor impurity metal, wherein said second III-Vlayer is a p-type III-V layer; resuming said step of transporting saidsecond halide metal compound to said growth zone; discontinuing saidstep of transporting said at least one acceptor impurity metal to saidgrowth zone; and growing a third III-V layer on said second III-V layer,said third III-V layer formed by said reaction gas reacting with saidfirst and second halide metal compounds, wherein said third III-V layeris an n-type III-V layer.
 34. The method of claim 33, further comprisingthe steps of: depositing a first contact on said third III-V layer; anddepositing a second contact on said substrate.
 35. The method of claim33, further comprising the steps of: discontinuing said step oftransporting said second halide metal compound to said growth zone; andgrowing a fourth III-V layer on said third III-V layer, said fourthIII-V layer formed by said reaction gas reacting with said first halidemetal compound, wherein said fourth III-V layer is an n-type III-Vlayer.
 36. The method of claim 35, further comprising the steps of:depositing a first contact on said fourth III-V layer; and depositing asecond contact on said substrate.